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APL gets more solar surprises from NASA's Voyager

Data from NASA's Voyager 1 spacecraft continue to provide new insight into the outskirts of our solar system, a frontier thought to be the last that Voyager will cross before becoming the first man-made object to reach interstellar space.

In papers published in June in the journal Science, scientists from Johns Hopkins' Applied Physics Laboratory and other Voyager partner institutions provide more clarity on the region they named the "magnetic highway" in December 2012. Cruising through what scientists describe as a curious, unexpected charged-particle environment, Voyager has detected, for the first time, low-energy galactic cosmic rays, now that particles of the same energy from inside the bubble around our sun have disappeared. As a result, Voyager now sees the highest level so far of particles from outside our solar bubble that originate from the death of other nearby stars.

"Voyager 1 may be months or years from leaving the solar system—we just don't know," says APLís Stamatios Krimigis, principal investigator for Voyager's Low-Energy Charged Particle, or LECP, instrument, which was designed and built at APL with NASA funding. "But the wait itself is incredibly exciting, since Voyager continues to defy predictions and change the way we think about this mysterious and wonderful gateway region to the galaxy."

Voyager 1 and 2 were launched in 1977 and between them have visited Jupiter, Saturn, Uranus, and Neptune. Since 1990, the twin spacecraft have been on their interstellar mission, on track to leave the heliosphere, which is the bubble of magnetic field and charged particles that the sun blows around itself. On Aug. 25, 2012, when Voyager 1 was about 11 billion miles from the sun, the spacecraft reached the magnetic highway, where charged particles from inside the heliosphere zoomed out along the magnetic field as cosmic rays from far outside zoomed in. The lack of a detectable change in the direction of that magnetic field convinced scientists that Voyager remained within the sun's influence.

The new Science papers focus on observations from summer and fall 2012 by LECP as well as Voyager 1's Cosmic Ray and Magnetometer instruments, with additional LECP data through April 2013.

"The most dramatic part was how quickly the solar-originating particles disappeared; they decreased in intensity by more than 1,000 times, as if there was a huge vacuum pump at the entrance ramp onto the magnetic highway," Krimigis says. "We have never witnessed such a decrease before, except when Voyager 1 exited the giant magnetosphere of Jupiter some 34 years ago."

"Surprisingly, the traveling direction of the 'inside' charged particles in this region made a difference, with those moving straightest along the magnetic field lines decreasing most quickly. Those that moved perpendicular to the magnetic field did not change as quickly," says LECP co-investigator Robert Decker, also of APL. The cosmic rays from outside, moving along the field lines, were somewhat more intense than those moving perpendicular to the field, and this imbalance varied significantly with time during the eight months since. Adds APL's Edmond Roelof, also an LECP co-investigator, "It is this time-varying behavior of the cosmic rays that tells us that we're still in a region controlled by our sun."

The multidimensional measurements speak to the unique abilities of the LECP detector, designed at APL in the 1970s. It includes a stepper motor that rotates the instrument through 45-degree steps every 192 seconds, allowing it to gather data in all directions and pick up something as dynamic as the solar wind and galactic particles. The device, designed and tested to last four years and work for 500,000 steps, has been working for nearly 36 years and well past 6 million steps.

Voyager 1 is 11.6 billion miles from the sun, poised to become Earthís first robotic ambassador to the space between the stars. At 9.4 billion miles, Voyager 2 has seen some gradual changes in the charged particles, yet scientists do not think Voyager 2 has reached the magnetic highway.

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